This invention relates to a simple means for reducing the abrasivity of abrasive pigments such as calcined clay and titanium oxide pigments used by the paper industry.
Calcined kaolin and titania pigments have been used for several decades in a number of industrial applications such as paper coating, paper filling, paints, plastics, etc. In those applications they impart to the finished products a number of desirable properties: brightness, opacity and hiding power. In the case of calcined kaolin pigments, paper coating and filling applications require almost exclusively ultrafine fully calcined kaolin pigments such as Ansilex 93.RTM. pigment manufactured by Engelhard Corporation. See, for example, U.S. Pat. No. 3,586,523, Fanselow, et al., the teachings of which are incorporated herein by cross-reference.
These fully calcined pigments have an undesirable property, namely, they are relatively abrasive when compared to noncalcined (hydrous kaolin pigments) or, in some cases, partially calcined (metakaolin) pigments. For example, conventional so-called "low abrasion" calcined kaolin pigments typically have an Einlehner abrasion value of about 20 mg. An ultrafine calcined kaolin pigment which has recently been introduced to the paper industry has a lower abrasion but appears to have lower scatter properties than that of the somewhat coarser but more abrasive products such as ANSILEX 93 pigment. In practical terms, increased abrasivity translates into increased wear of web forming screens (wires) on paper making machines, dulling of paper slitter knives, wear of printing plates when they come into contact with coated paper containing fine calcined pigments in the coating formulation, and, in general, wear of any surfaces coming into contact with these pigments. Titanium dioxide pigments are generally significantly more abrasive than fully calcined kaolin pigments.
The Einlehner abrasion test simulates the wear of the web forming screens, other paper-making machine wear and printing plate wear; while another test, the filled-sheet needle abrasion test simulates wear on slitter and knife wear in the paper finishing and converting operations.
Paper makers are becoming increasingly demanding in their need for lower abrasion. To overcome the problem, suitable kaolin can be calcined at temperatures less than those required to produce fully calcined pigments. In this instance, calcination temperature is controlled to produce the form of pigment known as metakaolin. It is known, however, that the brightness of a metakaolin pigment is always poorer, generally by about 2-3%, than that of fully calcined pigments derived from the same clay calciner feed. However, even metakaolin pigments may be more abrasive than paper makers may desire. Examples of patents disclosing calcination of kaolins to provide pigments include: U.S. Pat. No. 3,586,523, Fanselow, et al.; U.S. Pat. No. 3,014,836, Proctor; U.S. Pat. No. 3,058,671, Billue; U.S. Pat. No. 3,343,943, Billue; U.S. Pat. No. 3,171,718, Gunn, et al.; U.S. Pat. No. 4,381,948, McConnell, et al. and U.S. Pat. No. 5,112,782, Brown, et al. Many of these patents make reference to the desirability of reduced abrasivity but in all cases achieve such result by kaolin crude selection and/or processing conditions utilized in steps carried out prior to the final calcination of the original hydrous kaolin feed.
U.S. Pat. No. 4,678,517, Dunaway; U.S. Pat. No. 4,830,673, Jones, et al.; and U.S. Pat. No. 5,022,924, Raythatha, et al. disclose the addition of: particulate oxidizable materials, particularly sawdust; aliphatic diols; and colloidal or fumed silica, respectively, to clays prior to calcination in order to obtain performance benefits, including reduced abrasivity, in the calcined clay.
Originally kaolin pigments were supplied in dry form to the end users. The demand for pigments in aqueous slurry form developed with the availability of appropriate shipping and handling facilities. Since the cost of shipping water is an undesirable expense, it is desirable to provide pigment slurries as concentrated as is feasible, consistent with the necessity of formulating slurries that are sufficiently fluid to be pumped yet are resistant to settling. This posed no significant problem when formulating slurries of hydrous (uncalcined kaolins); using conventional anionic clay dispersants such as condensed phosphate salts and conventional clay handling equipment, 70% solids slurries of fine particle size coating grade hydrous kaolin were readily achieved.
Slurries of fine particle size calcined kaolin pigments containing about 50% solids have been commercially available for several years and require the use of organic polymers to prevent sedimentation. The "normal" procedure of making such slurries of calcined kaolin slurries is first to make an aqueous solution of a thickening agent, such as CMC (carboxymethyl cellulose), in water containing a microbicide, then add a dispersant, such as C211 (sodium polyacrylate) and mix the slurries in a high shear mixer.
Patents related to the stabilization and rheology enhancement of aqueous slurries of calcined kaolin include U.S. Pat. No. 5,034,062, Lein, et al., U.S. Pat. No. 5,028,268, Ince, et al., U.S. Pat. No. 4,017,324, Eggers, and U.S. Pat. No. 3,846,147, Tapper.
Other patents relating to the production of slurries of calcined kaolin clay or calcined pigments derived from kaolin clay are U.S. Pat. No. 3,582,378, Miller and U.S. Pat. No. 5,006,574, Sennett, et al., both assigned to the assignee of the subject patent application.
Applicant's copending applications, U.S. Ser. No. 07/898,116 and U.S. Ser. No. 07/971,006 (both also assigned to the assignee of the instant application) relate to the use of various surfactants to reduce abrasion.
The present application relates to the use of polytetrafluoroethylene (PTFE) resin particles to reduce Einlehner abrasion.
The use of PTFE resins to control dusting of fine powders is disclosed in U.S. Pat. Nos. 3,838,064, 3,838,092, 3,974,089 and 3,993,584. After the PTFE resin is added, it is subjected to treatment intended to fibrillate the resin, resulting in a random network of PTFE fibers distributed throughout the fine powder.